While these sources list polystyrene as having a max rating of about
160F, which sounds fairly high, I would
never use it
inside a collector -- ever! Use polyisocyanurate in collectors --
most lumber yards stock this.

NIST Heat Transmission Properties
of Insulating
and Building Materials

An database of thermal conductivity
values for a wide range of materials. Not the easiest to use, but it
does provide conductivity measurements for a wide variety of materials all
tested using the same procedure.

The easiest way to use the database
appears to be go to the database search form, set the units you want,
pick the "Material" from the drop-down list, and search.